This invention relates to fuel cells, and, more particularly, to solid oxide fuel cells having thin electrolyte and interconnect layers and, consequently, low internal electrical resistance.
A fuel cell is a device in which a first reactant, a fuel such as hydrogen or a hydrocarbon, is electrochemically reacted with a second reactant, an oxidant such as air or oxygen, to produce a DC electrical output. A fuel cell includes an anode, or fuel electrode, which enhances the rate at which electrochemical reactions occur on the fuel side. There is also a cathode, or oxidant electrode, which functions similarly on the oxidant side. In the solid oxide fuel cell, a solid electrolyte, made of dense yttria-stabilized zirconia (YSZ) ceramic separates a porous ceramic anode from a porous ceramic cathode. The anode typically is made of nickelous oxide/YSZ cermet, and the cathode is typically made of doped lanthanum manganite.
In such a fuel cell, the fuel flowing to the anode reacts with oxide ions to produce electrons and water, which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and thence to the cathode. The electrolyte is a nonmetallic ceramic that is a nonconductor of electrons, ensuring that the electrons must pass through the external circuit to do useful work. However, the electrolyte permits the oxide ions to pass through from the cathode to the anode.
Each individual electrochemical cell, made of a single anode, a single electrolyte, and a single cathode, generates a relatively small voltage. To achieve higher voltages that are practically useful, the individual electrochemical cells are connected together in series to form a stack. The cells are connected in series electrically in the stack. The fuel cell stack includes an electrical interconnect between the cathode and the anode of adjacent cells. The fuel cell assembly also includes ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel cell is desirably made inexpensively, but with excellent output performance. For most applications, it is also desired that the fuel cell have a high output power-to-weight ratio. There is an ongoing need for fuel cell designs and fabrication techniques that achieve these ends. The present invention provides such an approach, and further provides related advantages.